1. Technical Field
This invention relates to an organic EL display apparatus comprising an organic EL device and a driving device and more particularly, to an organic EL display apparatus adapted to drive an organic EL device having an organic-inorganic junction structure using a thin film transistor having a non-single-crystal semiconductor thin film as a substrate.
2. Background Art
In the prior art, a technique of simple driving an organic EL display by a X-Y matrix circuit to perform image display is known (JP-A 2-37385 and 3-233891). However, such simple driving carries out line sequential driving. In the event that the scanning number is as many as several hundreds of lines, the required instantaneous luminance becomes several hundred times the observed luminance, and the following problems arise.
(1) The drive voltage becomes high. Since the voltage usually becomes at least 2 or 3 times the DC constant voltage, the efficiency drops. So the power consumption increases.
(2) Since the quantity of instantaneously conducted current becomes several hundred times, the organic light emitting layer is liable to deteriorate.
(3) Since a very large quantity of current conducts as in (2), the voltage drop across electrode wiring becomes significant.
As a measure for solving the above problems (1) to (3), the following active matrix driving was proposed. U.S. Pat. No. 4,143,297 discloses a display using an inorganic substance ZnS as a fluorescent substance and carrying out active matrix driving. This technique, however, suffers from the problem that the drive voltage is as high as 100 volts or above on account of the use of an inorganic fluorescent substance. A similar technique is described in IEEE Trans Electron Devices, 802 (1971). By contrast, there were recently developed many displays using organic fluorescent substances and carrying out active matrix driving (see JP-A 7-122360, 7-122361, 7-153576, 8-54836, 7-111341, 7-312290, 8-109370, 8-129359, 8-241047 and 8-227276). The above technique has many advantages. Using organic fluorescent substances, the drive voltage is drastically reduced to a low level of 10 volts or below. When high efficiency organic fluorescent substances are used, the efficiency is very high in the range of 3 to 15 lm/W. Also, the drive voltage of a high definition display becomes xc2xd to ⅓ as compared with the simple driving, achieving a reduction of power consumption. Despite these advantages, the following becomes a problem.
FIGS. 11 and 12 illustrate a prior art exemplary TFT drive circuit. Referring to these figures, one prior art exemplary active matrix is now described.
FIG. 11 is a panel block diagram in which a display panel 310 has a display screen 311, an X axis shift register 312, and a Y axis shift register 313.
The display screen 311 is powered by an EL power supply. The X axis shift register 312 is powered by a shift resister power supply and receives X axis synchronizing signals as the input. The Y axis shift register 313 is powered by a shift resister power supply and receives Y axis synchronizing signals as the input. The output section of the X axis shift register 312 has an output of image data signals.
FIG. 12 is an enlarged view of a portion delimited by circle A in FIG. 11. The display screen 311 has pixels, with one pixel (depicted by a broken line rectangular block) consisting of two transistors, one capacitor, and one EL device.
The radiative operation of one pixel is performed as follow. For example, when the Y axis shift register 313 delivers an address signal y1, the X axis shift register 312 delivers an address signal x1, transistors Ty11 and Tx1 are turned on.
As a result, an image data signal xe2x88x92VL is supplied to the gate of a drive transistor M11. This allows a current corresponding to the gate voltage to flow between the drain and the source of the drive transistor M11 from the EL power supply whereby an EL device EL110 emits light.
At the next timing, the X axis shift register 12 interrupts the supply of address signal x1 and instead, delivers an address signal x2, but the gate voltage of the drive transistor M11 is held by a capacitor c11. The light emission of the EL element EL110 is sustained until the same pixel is subsequently selected.
Consequently, even if the time allotted to one pixel is reduced by the increasing number of scanning lines, the driving of the organic EL device is not affected and the contrast of the image appearing on the display panel is not reduced. Therefore, the active matrix system enables to display images of significantly high quality as compared with the simple matrix system.
In the case of organic EL devices, the luminance of light emission depends on the current density applied because they are current-driven devices. When one intends to improve the luminance of light emission within the same pixel area, or when one intends, in the situation where the number of pixels is increased in order to improve the fineness of the overall screen so that the drive time (or time division interval) per pixel is reduced, to further increase the luminance of light emission to compensate for the reduced drive time, it is necessary to increase the drive current. In this regard, if one intends to increase the drive current, then the drive voltage is naturally increased although the relation varies depending on a particular circuit arrangement.
However, if one intends to construct a switching device adapted to accommodate high currents and high voltages, the off-state leakage current Ioff becomes increased, which causes erroneous light emission and a drop of contrast.
An object of the invention is to provide an organic EL display apparatus using an organic EL device capable of light emission to a high luminance at a relatively low voltage so that the apparatus is prevented from erroneous light emission and a drop of contrast and features a high operating speed.
This and other objects are accomplished by the following constructions.
(1) An organic EL display apparatus comprising
a switching device comprising a controlling electrode and a set of controlled electrodes formed on a non-single-crystal silicon substrate, and
an organic EL device adapted to be driven by the switching device and comprising a positive electrode, a negative electrode, and an organic layer between the electrodes participating in at least a light emitting function,
the organic EL device further comprising a high resistance inorganic electron injecting and transporting layer between the organic layer and the negative electrode capable of blocking holes and having conduction paths for carrying electrons, or a high resistance inorganic hole injecting and transporting layer between the organic layer and the positive electrode capable of blocking electrons and having conduction paths for carrying holes, or both.
(2) The organic EL display apparatus of (1) wherein the high resistance inorganic electron injecting and transporting layer contains
at least one oxide of an element selected from the group consisting of alkali metal elements, alkaline earth metal elements, and lanthanide elements, the oxide having a work function of up to 4 eV, as a first component and
at least one metal having a work function of 3 to 5 eV as a second component.
(3) The organic EL display apparatus of (1) or (2) wherein the second component is at least one element selected from the group consisting of Zn, Sn, V, Ru, Sm, and In.
(4) The organic EL display apparatus of (1) wherein the alkali metal element is at least one element selected from the group consisting of Li, Na, K, Rb, Cs, and Fr, the alkaline earth metal element is at least one element selected from the group consisting of Mg, Ca, and Sr, and the lanthanide element is selected from La and Ce.
(5) The organic EL display apparatus of (1) wherein the high resistance inorganic electron injecting and transporting layer has a resistivity of 1 to 1xc3x971011 xcexa9xe2x88x92cm.
(6) The organic EL display apparatus of (1) wherein the high resistance inorganic electron injecting and transporting layer contains 0.2 to 40 mol % based on the entire components of the second component.
(7) The organic EL display apparatus of (1) wherein the high resistance inorganic electron injecting and transporting layer has a thickness of 0.2 to 30 nm.
(8) The organic EL display apparatus of (1) wherein the high resistance inorganic hole injecting and transporting layer has a resistivity of 1 to 1xc3x971011 xcexa9xe2x88x92cm.
(9) The organic EL display apparatus of (1) wherein the high resistance inorganic hole injecting and transporting layer contains a metal and/or at least one member selected from the group consisting of an oxide, carbide, nitride, silicide and boride of the metal.
(10) The organic EL display apparatus of (1) wherein the high resistance inorganic hole injecting and transporting layer contains
an oxide of silicon and/or germanium as a main component, the main component being represented by (Si1xe2x88x92xGex)Oy wherein 0xe2x89xa6xxe2x89xa61 and 1.7xe2x89xa6yxe2x89xa62.2, and
a metal having a work function of at least 4.5 eV and/or at least one member selected from the group consisting of an oxide, carbide, nitride, silicide and boride of the metal.
(11) The organic EL display apparatus of (10) wherein the metal is at least one member selected from the group consisting of Au, Cu, Fe, Ni, Ru, Sn, Cr, Ir, Nb, Pt, W, Mo, Ta, Pd, and Co.
(12) The organic EL display apparatus of (10) wherein the content of the metal and/or the oxide, carbide, nitride, silicide and boride of the metal is 0.2 to 40 mol %.
(13) The organic EL display apparatus of (1) wherein the high resistance inorganic hole injecting and transporting layer has a thickness of 0.2 to 100 nm.
(14) The organic EL display apparatus of (1) wherein the switching device is a thin film transistor.
(15) The organic EL display apparatus of (1) wherein in the switching device, the non-single-crystal silicon substrate has an active layer with a thickness of 100 to 800 xc3x85.
(16) The organic EL display apparatus of (1) wherein the switching device has a S value of at least 0.8 V/decade.
(17) The organic EL display apparatus of (1) wherein the switching device has an off-current of up to 1xc3x9710xe2x88x928 A.